Later today, the website Climate Culture is releasing a list of the 2009 Top Green Myths, things that you do – or don't do – because you've read or been told they're good or bad for the environment – but which, surprisingly, may not be producing the green results you're expecting.

Lots of these have been argued before -- Is local food always greener? Are paper bags better than plastic? -- and there's not always one "right" answer to them. But let's look at the list and then get your opinion :

1. Green myth: Recycled paper is better for the environment than virgin paper. Fact: Recycled paper can sometimes be more carbon intensive than virgin paper. It depends on where you live. If your home is in the Pacific Northwest or Maine, where much of the electricity comes from hydro power, you may be better off with virgin paper since plants that manufacture recycled paper are often near large metro areas where power is from less efficient sources. The "difference in emissions from electricity use in paper production can be larger than the emissions associated with cutting down the tree to produce paper in the first place," notes Zeke Hausfather, executive vice president of energy science at Climate Culture.

2. Green myth: Local food is always greener. Fact: "The method of production and type of food is far more important than the distance traveled in determining life-cycle greenhouse-gas emissions. For example, chicken from the supermarket is likely greener than local beef from the farmer's market." That said, there are plenty of other reasons to buy locally produced food, Mr. Hausfather admits.

3. Green myth: Washing dishes by hand uses less water than a dishwasher. Fact: It depends. Often, people underestimate how much hot water they use when washing dishes by hand. The most environmentally friendly way: washing your dishes in cold water.

4. Green myth: It's better to drive to your vacation destination than to fly. Fact: Not if your car is an SUV, station wagon, minivan, or truck. That may be mitigated, though, if you have the entire family in the car, or drive a car that's fuel-efficient.

5. Green myth: Compact fluorescent lights (CFLs) result in mercury emissions; incandescents don't. Fact: "CFLs generally result in less mercury emissions than conventional incandescents, since coal-based electricity generation is the single largest source of anthropogenic mercury emissions and CFLs save a considerable amount of electricity," says Hausfather. While much has been made of the mercury dangers of broken CFLs, he notes that most of the bulb's mercury is bound to the glass.

6. Green myth: Given a choice between paper bags and plastic bags, go with paper. Fact: From a standpoint of carbon emissions, they're equally bad. Plastic is worst from a solid waste perspective. (But plastic is a littering problem in many places.) Most environmentally friendly of all, as you already know, is bringing your own reusable bags (which is, admittedly, easier if you aren't buying groceries for a family of four).

7. Green myth: An electric car is best for the environment. Fact: If you live in a state where most of your electricity is generated by coal, that's not so. In those areas, electric cars can emit more carbon than high-efficient hybrids. Unless the electricity for the car is generated solely by renewable energy, electric vehicles are "far from zero emissions."

8. Green myth: If you want to help alleviate global warming, plant trees. Fact: Once again, it depends on where you live. In areas with cold winters, "the additional sunlight absorbed by the dark-colored trees just about offsets any cooling from carbon reduced." In high-latitude regions, "planting trees can actually heat up the earth," Hausfather says. However, in urban areas and the tropics, planting trees is good from a global-warming perspective. (Remember, this is talking only about benefits to the climate, not to trees or other ecosystem effects.)

9. Green myth: Buy milk in paper or glass cartons if you have the choice. Fact: Because half-gallon plastic milk jugs use much less material, they have lower life-cycle greenhouse-gas emissions than glass or paper containers of the same size.

10. Green myth: Using your garbage disposal isn't good for the environment. Fact: It depends on a couple of local factors and also on what you would do with the garbage if you didn't put it in the disposal. If you're going to toss it in the trash, it's probably better to grind it up in the disposal, although the benefits may depend on how your community captures methane emissions from wastewater treatment and landfills. If you want to do it right, compost your leftover food.

I asked Hausfather to provide some scientific evidence for these claims and here they are:

1. Recycled versus virgin paper. This one is a bit thorny, he admits. He's willing to discuss it with me, so if you'd like to know more of what's behind the issue, say so in a comment and it can be a future blog post.

2. Local food is always green. This is based on a life-cycle analysis of food sources by Weber and Matthews (2008) in which they find:

"...the GHG emissions associated with food are dominated by the production phase, contributing 83% of the average U.S. household’s 8.1 t CO2e/yr footprint for food consumption. Transportation as a whole represents only 11% of life-cycle GHG emissions, and ﬁnal delivery from producer to retail contributes only 4%. Different food groups exhibit a large range in GHG-intensity; on average, red meat is around 150% more GHG- intensive than chicken or ﬁsh. Thus, we suggest that dietary shift can be a more effective means of lowering an average household’s food-related climate footprint than “buying local.” Shifting less than one day per week’s worth of calories from red meat and dairy products to chicken, ﬁsh, eggs, or a vegetable-based diet achieves more GHG reduction than buying all locally sourced food."

Their paper is available here. Note that this is a rather carbon-myopic approach, and there are plenty of benefits to local food beyond their associated GHG reductions.

3. Dishwashers versus hand-washing dishes: This is based on calculations using dishwasher efficiencies from the US Federal Trade Commission and a study by the British nongovernmental organization Waterwise. Average dishwasher age is estimated to be seven years, based on the Waterwise study. The relationship between energy use per cycle and water use per cycle is based on calculations by the US Department of Energy. Average water use and temperature for handwashing (80 minutes and 80 liters per 12 place settings, 90F) is derived from the study by Bonn University, in Germany. Dishwasher capacity is assumed to be12 place settings. Whether dishwashing or hand-washing is optimal will differ based on the fuel mix of electricity generation, the water temperature used in hand-washing, the user’s water heater fuel and efficiency, the efficiency of the dishwasher, the temperature setting of the dishwasher, the load factor of the user’s dishwasher, and the flow rate of the user’s kitchen sink (which we assume to be 2.5 gallons per minute based on average sink flow data from the Laurence Berkeley National Labs). There is no clear winner, other than washing by hand with cold water.

4. Flying versus driving the car on vacation: The calculations are based on the carbon emissions per mile from flying via the World Resources Institute. Vehicle emissions per mile are based on the highway m.p.g. of the vehicle in question using the EPA’s fuel economy database and the number of passengers in the vehicle, taking into account the effect on fuel economy of extra passenger weight.

5. Mercury emissions of CFLs versus incadescent lights: This is based on CFL mercury data from the EPA’s Energy Star Program. Mercury emissions per kWh are based on NERC subregion data from the EPA’s eGRID and state-level transmission loss data from the US Department of Energy. Health impacts of broken CFLs in homes based on remarks from Dr. Helen MacIntosh, a professor of environmental health at Harvard University, reported here.

6. Paper versus plastic: Paper and plastic bags both require comparable amounts of energy per bag for production, given that paper bags are considerably more massive than plastic ones, though paper bags are slightly preferred. Data on life-cycle carbon emissions for paper and plastic are taken from FRIDGE: Socio-economic impact assessment of the proposed plastic bag regulations. Other reports argue that paper bags have higher life-cycle GHG emissions, though methodologies and analysis boundaries differ across reports.

7. Electric cars: This is based on wheel-to-wheel efficiency data for Tesla Roadster and Toyota RAV4 electric vehicles and electricity generation emissions data from the EPA’s eGRID for each NERC subregion.

8. Planting trees to help alleviate global warming: There is a wide and contentious literature on this question, though most people agree that carbon benefits of afforestation in temperate areas is at least partially offset by albedo effects. One of the seminal papers on the subject is Bala et al (2007), where they argue that:

"...[in high latitude areas] the warming carbon-cycle effects of deforestation are overwhelmed by the net cooling associated with changes in albedo and evapotranspiration. Latitude-specific deforestation experiments indicate that afforestation projects in the tropics would be clearly beneficial in mitigating global-scale warming, but would be counterproductive if implemented at high latitudes and would offer only marginal benefits in temperate regions. Although these results question the efficacy of mid- and high-latitude afforestation projects for climate mitigation, forests remain environmentally valuable resources for many reasons unrelated to climate."

9. Plastic milk jugs versus paper or glass: This is based on a the revised version of the comprehensive life-cycle analysis of plastic, paper, and glass half-gallon milk containers from Franklin Associates. You can find a summary of the report here.

10. Using the garbage disposal: Composting is ideal, but using the garbage disposal can be better from a carbon perspective than landfilling organic waste if your local wastewater treatment plant captures its methane emissions. This will differ by geographic area, and there are no good publicly available databases on which wastewater treatment plants capture methane and which do not. Note that there are cases in which waste disposed through the sink does not go to the sewer but is discharged to streams and rivers, and food waste can contribute to eutrophication and other nasty (though not carbon related) ecosystem impacts. See here and here for two contrasting views on this issue.